Anatomy, photochemical activity and DNA polymorphism in leaves of dwarf tomato irradiated with X-rays

The response of higher plants to ionising radiation depends on factors related to both radiation properties and plant features including the species, cultivar, age, and structural complexity of the target organ. Adult plants of dwarf tomato were irradiated with different doses of X-rays to investigate the possible variations in leaf morpho-anatomical traits, photosynthetic efficiency, and genomic DNA. In order to assess if and how responses depend on leaf developmental stage, we analysed two types of leaves nearly mature leaves (L1) and actively developing leaves (L2), whose lamina size corresponded to 70 and 25 %, respectively, of the lamina size of fully expanded leaves. Results showed that X-rays prevented full lamina expansion of L2 leaves at all doses and induced early death in tissues of plants irradiated with doses higher than 20 Gy. Most anatomical modifications were not clearly dose-dependent and the radiation-induced increase in phenolic compounds was irrespective of dose. At high doses of X-rays (50 and 100 Gy), photochemical efficiency decreased significantly in both leaf types, whereas total chlorophyll content was significantly decreased only in L2 leaves. The random amplification of polymorphic DNA data showed that X-rays induce mutagenic effects in L2 leaves even at low doses, despite the absence of severe phenotypic alterations. The genetic structure found in the population of samples corroborates the results of anatomical and eco-physiological analyses: the 20 Gy dose seems to mark the threshold dose above which genetic alterations, structural anomalies, and perturbations in the photosynthetic apparatus become significant, especially in actively expanding leaves.